1. Field of the Invention
The present invention relates to a rotary stencil printer, and more particularly to a rotary stencil printer having a pinch roller or rollers adapted to press at least one of opposite side edge portions of a sheet discharged from a nipping region between a printing drum and a back press roller against the back press roller such that the sheet moves for a while as attached onto the back press roller.
2. Description of the Prior Art
In the stencil printing art according to which ink is supplied on one surface of a stencil sheet in the form of a layer, then the stencil sheet is laid on a surface to receive printing so as to be in contact therewith at the other surface thereof, and then a pressing force is applied to the ink layer by a pressing means such that the ink of the ink layer is transferred from said one surface to said other surface of the stencil sheet thereby giving an ink image on the surface to receive printing, it has been proposed by Japanese Patent Laid-open Publication 4-361043 to accomplish such a stencil printing in a manner of hardly causing a back inking of the printed sheets even when they are laid one over the other just after the printing according to such an art that after the ink has been transferred through the perforations of the stencil sheet, the printed surface is detached from the stencil sheet while the pressing means is still kept in its pressing position such that the ink of the ink layer is substantially kept under the control of the pressing means against flowing relative to the stencil sheet, whereby when the printed sheet is removed from the stencil sheet, the ink is retained on the stencil sheet not to be drawn out of the ink layer due to the viscosity of the ink, thereby preventing any excessive ink from being transferred onto the printed surface.
In the above-mentioned publication, there is shown an embodiment for carrying out such a stencil printing art in a rotary stencil printer, according to which a printing sheet nipped between a printing drum bearing a perforated stencil sheet around a cylindrical circumferential surface thereof and a back press roller and applied with a printing at the nipping region under an action of an inking roller acting as a means for pressing ink from the inside of the printing drum to thereby urge the ink radially outwardly of the printing drum is moved for a while as attached onto a cylindrical circumferential surface of the back press roller such that the printing sheet moving along an arcuate contour of the movement of the circumferential surface of the back press roller is detached from the stencil sheet wrapped around the printing drum at an early stage while the ink layer of the printing drum is still kept under the control of the inking roller with the ink being substantially prohibited of any free flow thereof, and as a means for transferring the printing sheet discharged from the nipping region as so attached to the outer circumferential surface of the back press roller there are provided a damp for fastening the leading edge of the printing sheet to the outer circumferential surface of the back press roller along a generatrix thereof and pinch rollers adapted to press opposite side edge portions of the printing roller against the outer circumferential surface of the back press roller.
The rotary stencil printer schematically shown in the above-mentioned laid-open publication has a construction that the printing drum and the back press roller have the same diameter as one another. When the printing drum of such a rotary stencil printer is constructed to have a perforated flexible circumferential wall which can slightly bulge radially outwardly at a portion thereof extending along a generatrix thereof when the portion is pressed radially outwardly by an inking roller arranged along the inside surface thereof in parallel with said generatrix as shown in Japanese Patent Laid-open Publication 1-204781, there is obtained an advantage that the distance between the central axes of the printing drum and the back press roller may be fixed at such a constant value that provides a small clearance between the flexible circumferential wall of the printing drum and the back press roller when the flexible circumferential wall of the printing drum is not bulged out by the inking roller so that the switching over between an operating condition in which the printing drum and the back press roller nip a printing sheet at a nipping region therebetween for carrying out printing and a non-operating condition in which no printing sheet is supplied between the printing drum and the back press roller and therefore the back press roller must be removed off from the printing drum so as not to be contaminated with the ink of the printing drum by a direct contact therewith need not be carried out in the manner of changing the distance between the central axes of the printing drum and the back press roller as is in the conventional printer having a rigid printing drum but can be carried out in a manner of controlling the selective bulging out of the flexible circumferential portion of the printing drum by the inking roller.
As is shown in the above-mentioned Japanese Patent Laid-open Publication 1-204781, the printing drum having the above-mentioned flexible circumferential wall is in fact so constructed that a rectangular perforated sheet such as a net material to provide the flexible circumferential wall is rolled with opposite side edge portions thereof being seated around a pair of annular members firmly connected with one another by a transverse bar bridged therebetween so as to provide opposite end portions of the printing drum, said transverse bar also serving as a means for mounting the leading edge of the stencil sheet wrapped around the circumferential surface of the printing drum. And further, in order to meet with the matter that the printing drum exhibits an outer configuration having a portion somewhat radially outwardly projecting from the cylindrical contour of the circumferential wall at the part of the transverse bar, the back press roller is formed with a transverse groove extending along a generatrix thereof at a part thereof which encounters with the transverse bar of the printing drum when it is rotated in synchronization with the printing drum in a rotational direction opposite to that of the printing drum.
When the opposite side edge portions of a printing sheet are pressed against the outer circumferential surface of the back press roller by the above-mentioned pinch rollers, it is required that the position of the pinch rollers is precisely determined such that they are each aligned to a narrow region extending along a side edge of the sheet so as not to contact and damage the just printed print image. Therefore, when the width of the printing sheet changes, the position of the pinch rollers along the axis of the back press roller must be readjusted. However, since the pinch rollers are pressed against the outer circumferential surface of the back press roller under a certain elastically pressing force so that the printing sheet is tightly attached against the outer circumferential surface of the back press roller, shifting of the pinch rollers relative to the back press roller along the axis thereof will be resisted by a relatively large friction force.